This invention relates to the protection of insulated gate transistors from electrostatic charges, particularly insulated gate transistors fabricated of silicon carbide semiconductor material.
A common semiconductor device is known generally as an insulated gate field-effect transistor (IGFET), and a more particular example is a metal-oxide-semiconductor field-effect transistor (MOSFET). A MOSFET advantageously has a high gate impedance, and such devices are used in a wide variety of applications, ranging from both digital and analog integrated circuits in which a multiplicity of individual MOSFETs are formed on a single semiconductor "chip" and comprising a useful device such as an amplifier, to large power semiconductor devices comprising a single transistor per device.
In a typical MOSFET transistor structure, a gate electrode comprises a metal layer formed over a relatively thin layer of gate oxide, which serves as an electrical insulator between the gate electrode and the rest of the device. A well-known disadvantage of this device structure is that, in view of the high gate impedance, casually generated electrostatic charges can easily produce voltages higher than that which the insulating gate oxide can withstand, resulting in rupture of the gate oxide and consequent destruction of the device.
Such device destruction is primarily of concern during device handling, transport, or the like, prior to being installed within an actual circuit. For this reason, appropriate handling precautions are routinely employed, such as grounding of persons and equipment that are likely to contact MOSFET semiconductor devices during handling, and use of conductive packaging which tends to provide shunt current paths between the various terminals of a semiconductor device package. In most cases, once a device is installed within a circuit, other circuit elements serve to minimize the opportunity for destructive static charge buildup.
Gate protection is conventionally provided within MOSFET semiconductor device structures, particularly in silicon semiconductor devices, by including integral protection devices, such as avalanche (Zener) diodes which are reverse biased during normal circuit operation, and yet conduct when excessive voltage is applied so as to prevent damage to the gate oxide layer.
Other semiconductor device materials, however, are not amenable to conventional techniques for providing gate oxide protection structures, such as, for example, amplifiers and other integrated circuits which employ silicon carbide (SIC) semiconductor material. Advantageously, SiC is a crystalline substance that can withstand very high temperatures. Semiconductor devices manufactured on SiC substrates can withstand temperatures in excess of 200.degree. C. Thus, SiC based semiconductors are desirable for applications that require exposure to high temperatures, such as gas turbine control circuits. SiC, however, presents a number of fabrication difficulties, some of which are addressed in Krishnamurthy et al. U.S. patent application Ser. No. 08/201,494, filed Feb. 24, 1994, entitled "Silicon Carbide Integrated Circuits", assigned to the instant assignee and now U.S. Pat. No. 5,385,855. Another fabrication difficulty, which particularly relates to gate protection, is that it is difficult to form Zener diodes in SiC that are effective for gate protection. While it is possible to fabricate a zener diode in a SiC semiconductor device, the Zener breakdown voltage is typically so high (for example 500 volts or more) that insulated gate protection is ineffective. Thus, the gate oxide or other gate insulation breaks down before the Zener avalanche voltage is reached.